Drugs are most often currently administered orally by the ingestion of tablets, capsules or aerosols, or via subcutaneous, intramuscular or intravenous injections or implants. Oral solid dosage forms account for 40-50% of the market, parenteral products—33% and the other more “novel” dosage forms (NDF's), only a few %. There is nonetheless enormous perceived potential for NDF's that can enhance a drug's therapeutic ratio and avoid patient non-compliance. Non-compliance remains a major issue despite 95% of patients being aware of its consequences. Common examples are incomplete courses of antibiotic therapy, using antidepressive drugs for too short a period, and forgetting to take contraceptive pills.
There are known implants that are implanted subcutaneously and which deliver a drug over a period of time in a controlled manner. These are typically based on polymer material systems. There are two basic types of implant for controlled drug delivery; “reservoir” and “monolithic” structures. “Reservoir” devices have layers which are corroded or absorbed by the body to release a depot of drug beneath these control layers. By having successive alternate control layers and drug layers the drug can be released over a period of time. “Monolithic” devices have the drug distributed throughout, so that release kinetics are controlled by slow corrosion and diffusion processes.
Problems include the so-called “burst effect” wherein an unwanted high fraction of the drug is released from the polymer capsule's internal surface quite soon upon in-vivo exposure. Another problem is the continuing need for high-purity, cost effective hosts that are capable of sustained drug delivery over months or years (for some applications).
Other known implants include inert ceramic implants which have the drug held in their pores, the drug having to leave the ceramic implant via a tortuous path of micropores, which delays its release and allows it to be controlled.